Active noise cancelation (ANC) is a conventional method of reducing an amount of undesired noise received by a user listening to audio through headphones. The noise reduction is typically achieved by playing an anti-noise signal through the headphone's speakers. The anti-noise signal is an approximation of the negative of the undesired noise signal that would be in the ear cavity in the absence of ANC. The undesired noise signal is then neutralized when combined with the anti-noise signal.
In a general noise-cancelation process, one or more microphones monitor ambient noise or residual noise in the ear cups of headphones in real-time, then the speaker plays the anti-noise signal generated from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the headphone, frequency response of the speaker and microphone transducers, latency of the speaker transducer at various frequencies, sensitivity of the microphones, and placement of the speaker and microphone transducers, for example.
In feedforward ANC, the microphone senses ambient noise but does not appreciably sense audio played by the speaker. In other words, the feedforward microphone does not monitor the signal directly from the speaker. In feedback ANC, the microphone is placed in a position to sense the total audio signal present in the ear cavity. So, the microphone senses the sum of both the ambient noise as well as the audio played back by the speaker. A combined feedforward and feedback ANC system uses both feedforward and feedback microphones.
Along with reducing the ambient noise heard by a user, however, ANC systems also add a small amount of noise. This added noise may be noticeable to the user as a hiss when the user is in a quiet environment.
For example, FIG. 1 is a plot showing noise floor levels for three conditions of an example headphone having a conventional ANC system operated in a quiet room. A first trace 101 represents the noise floor over various frequencies when the headphone and the conventional ANC system are both powered off, which is the ambient noise level of the environment. A second trace 102 represents the noise floor when the headphone is powered on and the conventional ANC system is powered off. And a third trace 103 represents the noise floor when the headphone and the conventional ANC system are both powered on. Frequency is indicated on the horizontal axis, while the vertical axis indicates power spectral density.
As illustrated in FIG. 1, there may be a range of frequencies in which the noise floor with the headphone and conventional ANC system both powered on, shown in the third trace 103, exceeds the noise floor with the headphone and conventional ANC system both powered off, shown in the first trace 101. As noted, the user may perceive this as ANC hiss, particularly in a quiet environment.
Even when there is no ANC hiss, some users find strong ANC to be unpleasant.
Embodiments of the invention address these and other issues in the prior art.